Holographic techniques for storing images are well known. Such techniques are commonly used to store images of three dimensional objects in a wide variety of different applications. Additionally, various methodologies for utilizing such holographic techniques to store digital data for use in computer systems are currently being explored.
The technique for forming holograms comprises splitting the highly coherent output beam of a laser into separate reference and object beams. The reference beam is directed onto the holographic storage medium, e.g., a photorefractive material, while the object beam is directed onto the object whose image is to be stored. Light reflected from the object is directed to the photorefractive material wherein an interference pattern is formed via interference of the reference beam with the reflected light of the object beam. In the case of digital data storage, the object beam may pass through an optical modulator rather than being reflected off the object whose image is to be stored.
Subsequently, directing a reference beam onto the holographic storage medium results in the reconstruction of an image representative of the originally illuminated object or digital data.
Also known are techniques for storing a plurality of such images within a single photorefractive medium via angle-multiplexing of the images. Such angle-multiplexing is discussed in "THEORY OF OPTICAL INFORMATION STORAGE IN SOLIDS," Applied Optics, Vol. 2, No. 4, p. 393 (1963). The method of angle-multiplexing generally involves maintaining a constant angle for the object beam, while varying the angle of the reference beam for each exposure. Anglemultiplexing thus allows a large number of holographic images to be stored within a common volume of photorefractive medium, thereby greatly enhancing the storage density thereof.
In order to provide increased total storage capacity, it is, in general, desirable to utilize multiple holographic storage modules, wherein each storage module comprises a separate storage medium. The use of such multiple storage modules, however, presents technical problems which must be overcome in order to make such construction feasible. One problem commonly associated with the use of multiple storage modules is the ability to quickly and accurately address the desired hologram stored within the storage medium of a selected storage module. Not only must the reference beam be directed to the selected storage medium, but the reference beam must be directed thereto in a manner which provides a sufficiently flat wavefront within the reference beam plane of the selected storage medium to avoid inadvertent, partial recall of other holograms stored within said storage medium.
The degree of flatness required is determined by the angular separation between adjacent multiplexed holograms as discussed in detail below.
As such, it is desirable to provide a means for addressing a plurality of different storage modules and means for adjusting the wavefront flatness of the addressing reference beam, preferably automatically, each time a different storage module is addressed.